1. Field of the Invention
The present invention relates to a detachable timing gear cassette unit, in particular of the chain type, which is in the form of an independent assembly mounted on one of the end surfaces of the engine for insuring synchronization between the crankshaft and the camshaft.
2. Description of the Prior Art
The driving of the camshaft in synchronism with the crankshaft is presently accomplished in any one of three main ways, namely by means of gears, a chain, or a notched belt.
Historically, the camshaft was driven by gear pinions, subsequently by a timing gear chain mounted between a sprocket rigidly attached to the crankshaft and a sprocket rigidly attached to the camshaft, in the case of an overhead camshaft engine. The timing gear chain travels between the two sprockets in cavities provided in the front of the crankcase, the cylinder block, and the cylinder head, which are the three major assemblies of the engine. The timing gear chain is lubricated by engine oil contained in the crankcase and it is, therefore, necessary to insure appropriate oil seals between the various surfaces of the crankcase, the cylinder block, and the cylinder head in the region of the front cavity in which the chain travels.
To insure an appropriate oil seal in the vertical plane parallel to the end surfaces of the engine, taking into account the machining tolerances of the three aforementioned assemblies, a re-machining of the front end surface of the cylinder head, the cylinder block and the crankcase is sometimes required. Naturally, re-machining is a costly phase in the mass production of the engine.
In order to overcome the complicated design of the cavity for the chain in the cylinder block, the cylinder head and the crankcase, and in particular to solve the costly sealing problems, it has been proposed to drive the camshaft by means of a drive system external to the engine. This type of system consists of a notched belt made from a reinforced elastomer. The notched belt has projections which cooperate with sprockets provided with corresponding cavities. The sprockets are rigidly attached to the end of the camshaft and the end of the crankshaft so that these two shafts are driven in synchronism.
This system is completely outside the engine and, consequently, is exposed to the exterior environment. In particular, the system is exposed to grease or oils present around an engine and to dust, insects, and water encountered as the vehicle travels. Consequently, clogging of the cavities of the sprockets on which the notched belt is engaged occurs. The cavities may, consequently, be partly filled with those external environmental substances resulting in the belt escaping from these cavities in the pulleys and in a consequential defect in the setting of the timing gear. The belt may even break when snow or ice is trapped between the belt projections and the cavities due to an increase in the pitch diameter.
Initial attempts have been made to overcome this difficulty by isolating the notched belt and its sprockets in a protective housing. This solution created further difficulties due to the normal heating of the system in the course of its operation which resulted in temperatures unsuitable for the proper operation of the belt of elastomeric material.
Further, although the timing gear system using a notched belt has differences in noise of operation as compared to a transmission by chains, it must be noted that these notched belts have a certain fragility and a much shorter effective life than conventional chains. Moreover, in order to withstand the reaction torques produced in the camshaft, sufficient structure must be given to the notched belts and to the corresponding sprockets with which they cooperate so as to take into account the power of the engine. This additional structure, of course, increases the weight and overall size of the timing gear assembly.